Decorative laminates are generally constructed from plural layers of synthetic resin impregnated paper sheets bonded under heat and pressure to form a unitary structure. With regard to the manufacture of high pressure laminates, the decorative laminate sheet assembly includes at least a core of one or more phenol formaldehyde resin impregnated sheets and a melamine impregnated decorative sheet. The decorative sheet may be further covered with a transparent melamine impregnated overlay layer.
The decorative sheet provides the laminate with an attractive appearance. The decorative sheet, and the overlay layer, dictate the surface characteristics of the decorative laminate. For example, the composition of the decorative sheet and overlay layer dictates the decorative laminate's resistance to chemical agents, heat, light, impact and abrasion.
Decorative sheets are commonly manufactured from high quality 60-205 grams/meter2, pigment filled, alpha cellulose paper impregnated with a water alcohol or water solution of melamine-formaldehyde resin. The resin impregnated decorative sheets are subsequently dried, partially cured, and finally cut into sheets. The pigment filled, alpha cellulose paper of the decorative sheet, may include a solid color, a decorative design, or a roto-gravure reproduction of natural materials, such as, wood, marble, leather, abstract design, etc. The aesthetic characteristics of the cellulose paper are revealed as the laminate's decorative design upon completion of the manufacture of the decorative laminate.
The core, or base, functions to impart rigidity and impact resistance to the laminate. Prior to stacking, the paper sheets of the core are impregnated with a water alcohol or water solution of phenol formaldehyde resin, dried and partially cured in a hot oven. The cured laminate is then cut into predetermined shapes used to assemble the laminate layer. The core may, for example, include a plurality of sheets of 160 to 550 grams/meter2 basis weight Kraft paper, impregnated with phenol formaldehyde resin, wherein the phenol formaldehyde resin is generally 25% to 35% of the final weight of the impregnated or coated core sheet. The Kraft paper is impregnated throughout and bonded with a substantially cured phenol formaldehyde resin which has been converted to a thermoset state during the initial laminating step.
Those skilled in the art will appreciate that the overlay paper is commonly impregnated with melamine resin and can be positioned over decorative paper for improving the wear characteristics of the laminate. It is also commonly appreciated overlays impregnated with melamine resin can be positioned over decorative paper that is not impregnated with resin.
Impregnation of the paper and drying of the impregnated paper may be affected by conventional treaters and dryers known to those skilled in the art. The resin content of the impregnated paper generally ranges from about 55% to about 65% by weight based on the weight of the impregnated decorative paper, while the resin content of the overlay paper is generally high, for example, 65% to 82%. Paper volatile levels are maintained at about 5%+−0.5%. A balance between flow levels and treated weight of the treated paper is maintained to assure proper abrasion levels without reducing crack and/or craze resistance. Treated paper is beta-staged by drying at 65-182° C. for about 20-300 seconds and stored flat in a conditioned storage area. It has a nominal shelf life of at least about 3 months at 70-75° F. and 40-50% relative humidity.
A solid substrate may be bonded to the laminate where additional rigidity is required. Substrates are generally composed of a pre-cured plastic laminate, such as glass fiber-reinforced thermoset polyester resin laminates and the like; a wood product, such as hardboard, wood waste or particle boards, plywood and the like; a mineral base board, such as, cement-asbestos board, SHEETROCK® (plaster wall board), plaster board, and the like; or a combination of substrates. The substrate is commonly applied to the phenol formaldehyde resin impregnated sheets making up the core only after the laminate lay-up is pressed and heated to form the final decorative laminate.
Decorative laminates are generally manufactured by stacking the resin impregnated core sheets, the decorative sheet and the overlay layer to form a laminate sheet assembly, or laminate lay-up. The laminate lay-up is placed between steel plates and subjected to temperatures in the range of about 121° C.-160° C., preferably 131° C., and pressure of about 56.24 kg/cm2-112.48 kg/cm2, preferably 85 kg/cm2 for a time sufficient to consolidate the laminate and cure the resins (generally about 25 minutes to an hour). The pressure and heat force the resin in the paper sheets to flow, cure and consolidate the sheets into a unitary laminated mass referred to in the art as a high pressure decorative laminate. Finally, the formed decorative laminate is bonded to a reinforcing substrate, such as, plywood, hardboard, asbestos board, particle board or the like.
The melamine resin employed in both the decorative layer and the overlay melamine resin is clear and colorless when fully cured and has stain and wear characteristics that have made it the resin of choice for many commercially available laminates. Resistance to discoloration due to heat and light is also realized.
The above discussion generally relates to high pressure laminates. Where a less expensive, less durable laminate is required, low pressure laminates may be used by fabricators. Low pressure laminates are generally composed of only the decorative and overlay layers. As with high pressure laminates, low pressure laminates are bonded to a substrate for added structural rigidity at the time of manufacture. The removal of the core results in a laminate which is less expensive to manufacture, but does not offer the strength and durability of high pressure laminates.
The resins employed in the manufacture of conventional decorative laminates are relatively expensive and rely upon petroleum products. As such, manufacturers are continually looking for manufacturing methods that allow for the manufacture of decorative laminates in a more cost effective, environmentally friendly manner.